locking/lockdep: Make class->ops a percpu counter and move it under CONFIG_DEBUG_LOCKDEP=y

A sizable portion of the CPU cycles spent on the __lock_acquire() is used
up by the atomic increment of the class->ops stat counter. By taking it out
from the lock_class structure and changing it to a per-cpu per-lock-class
counter, we can reduce the amount of cacheline contention on the class
structure when multiple CPUs are trying to acquire locks of the same
class simultaneously.

To limit the increase in memory consumption because of the percpu nature
of that counter, it is now put back under the CONFIG_DEBUG_LOCKDEP
config option. So the memory consumption increase will only occur if
CONFIG_DEBUG_LOCKDEP is defined. The lock_class structure, however,
is reduced in size by 16 bytes on 64-bit archs after ops removal and
a minor restructuring of the fields.

This patch also fixes a bug in the increment code as the counter is of
the 'unsigned long' type, but atomic_inc() was used to increment it.

Signed-off-by: Waiman Long <longman@redhat.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will.deacon@arm.com>
Link: http://lkml.kernel.org/r/d66681f3-8781-9793-1dcf-2436a284550b@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 1 insertions, 6 deletions

diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index b0d0b51c4d85..1fd82ff99c65 100644
--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h
@@ -99,13 +99,8 @@ struct lock_class {
          */
         unsigned int                    version;
 
-        /*
-         * Statistics counter:
-         */
-        unsigned long                   ops;
-
-        const char                      *name;
         int                             name_version;
+        const char                      *name;
 
 #ifdef CONFIG_LOCK_STAT
         unsigned long                   contention_point[LOCKSTAT_POINTS];